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Hereditary and Congenital Malformations Congenital disease: abnormality present at birth, even though it may not be detected until some time after birth Hereditary or genetic disease: resulting from a chromosome abnormality or a defective gene

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Genetics (1 of 5) Genetics: study of heredity; transmission of physical, biochemical, and physiologic traits from biological parents to their children Disorders can be transmitted by gene mutations that can result in disability or death Genetic information is carried in genes strung together on strands of DNA to form chromosomes Except reproductive cells, every normal human cell has 46 chromosomes

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Genetics (2 of 5) Chromosomes: composed of double coils of DNA Genes: segments of DNA chains Genome: sum total of all genes contained in a cell’s chromosomes; the same in all cells In human beings, normal chromosome component: –22 pairs of autosomes –1 pair of sex chromosomes (XX in females and XY in males) Karyotype: a representation of a person’s set of chromosomes

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Genetics (3 of 5) In somatic cells: chromosomes exist in pairs, one member of each pair is derived from male parent and other from female parent –With 22 pairs called autosomes –Except for the sex chromosomes, members of the pair are similar in size, shape, and appearance (homologous chromosomes) Mitosis: cell division of somatic cells –Each of two new cells or daughter cells receives the same chromosomes as the parent cell

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Genetics (4 of 5) Not all genes are expressed in all cells and not all genes active all the time Meiosis: cell division that occurs during development of egg and sperm –Number of chromosomes is reduced –Daughter cells receive only half of chromosomes possessed by the parent cell DNA ultimately controls formation of essential substances throughout the life of every cell in the body through the genetic code (precise sequence of AT and CG pairs on the DNA molecule)

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Genetics (5 of 5) Genes control –Hereditary traits, cell reproduction, and daily functions of all cells –Cell function, through structures and chemicals made within the cell –RNA formation that controls formation of specific proteins; most are enzymes that assist in chemical reactions in cells

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Trait Predominance (1 of 2) Each parent contributes 1 set of chromosomes (or a set of genes) so that every child has two genes for every locus on the autosomal chromosomes Some characteristics or traits of the child are determined by 1 gene that may have many variants (e.g. eye color) Polygenic traits require interaction of ≥ 1 genes –Environmental factors may affect how genes are expressed Alleles: variations in a particular gene Homozygous: has identical alleles on each chromosome Heterozygous: alleles are different

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Trait Predominance (2 of 2) Children will express a dominant allele when one or both chromosomes in a pair carry it A recessive allele is expressed only if both chromosomes carry the recessive alleles For example, a child may receive a gene for brown eyes from one parent and a gene for blue eyes from the other parent –Gene for brown eyes is dominant –Gene for blue eyes is recessive –The dominant gene is more likely to be expressed and child is more likely to have brown eyes

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Autosomal Inheritance For unknown reasons, on autosomal chromosomes, one allele may be more influential than the other in determining a specific trait The more powerful or dominant gene is more likely to be expressed than the recessive gene

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Sex-Linked Inheritance X and Y chromosomes are not literally a pair, as X is much larger than the Y chromosome One X chromosome is inactivated (Lyon Hypothesis) –In males: one copy of most genes is on the X chromosome –Inheritance of these genes is called X-linked –A man will transmit one copy of each X-linked gene to his daughters and none to his sons –A woman will transmit one copy to each daughter or son

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Multifactorial Inheritance Environmental factors can affect the expression of some genes Example: child’s height will be within the range of height of both parents, but environmental factors such as nutritional patterns and health care also influence development –The better nourished, healthier children of two short parents may be taller than either

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Genetically Determined Diseases Result from abnormalities of individual genes on the chromosomes Chromosomes appear normal. Some defects arise spontaneously. Others may be caused by environmental teratogens (agents or influences that cause physical defects in the developing embryo) Mutation: permanent change in genetic material that may occur spontaneously or after exposure of a cell to radiation, certain chemicals, or viruses

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Chromosomal Abnormalities Nondisjunction: failure of homologous chromosomes in germ cells to separate in first or second meiotic division –May involve either sex chromosomes or autosomes –Causes abnormalities in distribution of chromosomes between germ cells –One of two germ cells has an extra chromosome while the other lacks a chromosome

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Chromosomal Abnormalities Monosomy: absence of a chromosome in a cell Trisomy: presence of an extra chromosome in a cell Deletions: chromosome breaks during meiosis and broken piece is lost Translocations: misplaced chromosome or part of it attaches to another chromosome

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Sex Chromosome Abnormalities (1 of 3) Variations in normal number of sex chromosomes are often associated with some reduction of intelligence –Y chromosome: directs masculine sexual differentiation, associated with male body configuration regardless of number of X chromosomes present –Extra Y: no significant effect as it mainly carries genes concerned with male sexual differentiation –Absent Y: body configuration is female –Extra X in female: has little effect (one X chromosome is inactivated) –Extra X in male: has adverse effects on male development

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Sex Chromosome Abnormalities (3 of 3) Fragile X Syndrome (x-linked mental deficiency) Not related to either excess or deficiency of sex chromosomes Associated with a characteristic abnormality of the X chromosome Second to Down syndrome as a major cause of mental deficiency

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Autosomal Abnormalities (1 of 3) Absence of an autosome results in the loss of several genes that development is generally not possible and the embryo is aborted. Deletion of a small part of an autosome may be compatible with development but usually results in multiple severe congenital abnormalities. Down syndrome: most common chromosomal abnormality, an autosomal trisomy

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Autosomal Abnormalities (2 of 3) With trisomy of small chromosome 21 Many fetuses are aborted early in pregnancy (70%) Those who live have Down syndrome –Nondisjunction during oogenesis occurs in 95% of cases Increased frequency with advancing maternal age: 1 in 50 if mother is > 40 years old Extra chromosome 21 acquired as part of the translocation chromosome

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Autosomal Abnormalities (3 of 3) Nondisjunction occurring in zygote –Most common chromosomal abnormality: 1:600 births –Manifestations: mental deficiency, cardiac malformation, major defects in other organ systems Trisomy of chromosome 13: Cleft lip and palate; abnormal development of skull, brain, and eyes; congenital heart defect; polydactyly Trisomy of chromosome 18: Both 13 and 18 trisomies are usually fatal in the neonatal period or in early infancy

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Effects of nondisjunction in meiosis, leading to formation of gametes with an extra or missing chromosome.

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Translocation Down syndrome (1 of 3) Occurs in small number of persons with Down syndrome Extra chromosome: chromosome 21 fused with chromosome 14 or another chromosome Total number of chromosomes not increased but genetic material is equivalent to 47 chromosomes Causes –Normal chromosomes in cells of both parents Translocation occurred accidentally during gametogenesis in the germ cells of one of the parents –14/21 carrier in one of the parents Carrier parent has only 45 chromosomes because one is the fusion of chromosome 21 with 14

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Translocation Down syndrome (2 of 3) 14/21 carrier in one of the parents Carrier parent is capable of transmitting abnormal chromosome to his or her children resulting in translocation Down syndrome

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Amniocentesis Alpha fetoprotein: high concentration in amniotic fluid is suggestive of a neural tube defect Amniotic fluid for study: transabdominal amniocentesis –Usually performed between the 14th and 18th week of pregnancy –Primary use: prenatal detection of chromosomal abnormality in women over age 35 due to higher incidence of Down syndrome in infants born to older women

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Amniocentesis

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Indications for Amniocentesis

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Chorionic Villus Sampling (1 of 2) Chorionic villi: frond-like structures that form part of placenta and attach to lining of uterus –Fetal cells are obtained for evaluation using chorionic villi sample –Amniotic fluid not used for analysis –Small catheter inserted through cervix to the site where villi are attached on the uterus –Small area is suctioned

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Discussion What are the consequences of chromosome nondisjunction? What is the karyotype of an individual with Down syndrome? Why does this happen? What are the possible outcomes of a pregnancy involving a 14/21 translocation chromosome? What are the causes of congenital abnormalities and their approximate incidences? What are the indications for amniocentesis?